The present invention relates to a satellite telecommunications system. In such a system a group of ground stations interchange information by means of a satellite. Each of these stations transmits information by modulation of a carrier frequency belonging to the particular station. When a first station wishes to listen to a second station, it is necessary to previously position the reception equipment of the first station on the appropriate carrier frequency. This carrier frequency is determined by adding to the carrier frequency on which the second station transmits, the frequency shift due to the passage via the satellite of the wave transmitted by the second station. This frequency shift is made necessary by the fact that a satellite, incorporating active components, cannot receive and retransmit on the same frequency.
As it is very difficult to have a stable reference frequency within a satellite, compared with a theoretical value, there is a certain inaccuracy with respect to this frequency shift, which is called drift. The drift also varies in time, particularly as a function of the orientation conditions of the satellite in space.
To ensure good reception of information transmitted by the different stations, it is necessary to ensure that the reception equipment of each station is able at all times to tune to the real frequency and not the theoretical frequency of the retransmission by the satellite of the various carriers, i.e. it must be aware of the satellite drift at all times.
For this purpose it is known to provide the reception equipment of the various stations with an automatic frequency control system (AFC system). However, the automatic frequency control cannot be used in satellite telecommunications systems such as the single channel per carrier or SCPC system, where the carrier is not transmitted continuously, but only when there is information to be transmitted.
In such systems, it is known to use a special station, called a central station, which has a perfectly stable oscillator, called the reference pilot, which transmits a reference frequency received by all the other stations, called peripheral stations. The latter are also provided with perfectly stable oscillators and are consequently at all times able to measure the satellite drift and therefore can be automatically tuned to the real retransmission frequency by the satellite of the different carriers.
However, such telecommunications systems have the disadvantage of requiring several perfectly stable oscillators in all the stations and are therefore expensive.